Projection system

ABSTRACT

The main object of the present invention is to present a projection system excellent in visibility from inside of projection system to outside, and capable of obtaining a sharp contrast display without being influenced by the ambient light. The present invention provides a projection system comprising: a projection screen, containing a substrate and a polarized light selective-reflection layer formed on the substrate and diffuse-reflects right-handed circularly polarized light or left-handed circularly polarized light, which displays an image by reflecting an image light irradiated from a projector; a projector for projecting an image light on the projection screen; and an external light screen installed at an external light entrance, disposed in a sequence of a linearly polarizing plate and a retardation plate from an external side, and circularly polarizes a light in reverse direction to the circularly polarized light diffuse-reflected by the polarized light-selective reflection layer, wherein a transmission axis of the linearly polarizing plate is in vertical direction.

BACKGROUND OF THE PRESENT INVENTION

1. Field of the Present Invention

The present invention relates to a projection system comprising aprojection screen, a projector, and an external light screen.

2. Description of the Related Art

A conventional projection system is generally designed to display anincident image light, from a projector, on a projection screen, and anobserver observes its reflected light as an image.

Recently, along with downsizing, price-reduction, etc. of the projectormain body, demand for use at home, such as home theater system, isincreasing, and the projection systems are widely used in generalhousehold. In such projection system, image contrast of an image isbrought forth by intensity difference of the incident light (imagelight), from the projector, incident on the projection screen. Forexample, when displaying a white picture on a black background, theportion of the projection screen lighten by the incident light will bewhite and the other portion will be black, thereby the image contrast isbrought forth by such difference in brightness between black and white.Therefore, to realize a favorable image display, it is required toemphasize the contrast difference by making the white display portionbrighter and the black display portion darker.

However, when such projection system is used at home, for example, it isoften placed in a living room, and such place is usually designed to getambient light such as external light. Accordingly, the projection systemreflects ambient light, such as external light, as well as the imagelight, and both white display portion and black display portion becomebrighter so that the difference of brightness between black and whitebecomes smaller. As a result, in such conventional projection screen, itis difficult to realize a favorable image display unless influence bythe ambient light such as external light is suppressed by using a meansto darken the room or other environmental means.

To solve such problem, it has been studied to develop a projectionscreen capable of realizing favorable image display even in the presenceof high levels of ambient light. For example, those using hologram areproposed. The projection screen using hologram has advantages that itcan make the white display portion brighter by controlling thescattering effect, it realizes a relatively favorable image display inpresence of high levels of ambient light, and it has wavelengthselectivity. However, it does not have polarized light selectivity, andthere is a problem that it can only display an image with sharpness oflimited degree. Further, there is a problem that, in the projectionscreen using hologram, it is difficult to obtain a wide screen becauseof manufacturing problem.

On the other hand, as a projection screen capable of realizing favorableimage display even in the presence of high levels of ambient light, aprojection screen using a polarized light selective-separation layer isproposed (see Japanese Patent Application Laid-Open Nos. 5-107660 and2002-540445). The projection screen using polarized lightselective-separation layer is capable of making the black displayportion darker while keeping the white display portion bright, and ascompared with ones using hologram, a sharp image can be displayed in thepresence of high levels of ambient light. Specifically, a cholestericliquid crystal, which reflects right-handed circularly polarized lightor left-handed circularly polarized light contained in image light, canbe listed. For example, when a cholesteric liquid crystal, whichreflects right-handed circularly polarized light, is used, theleft-handed circularly polarized light is not reflected by thecircularly polarized light separation function of the cholesteric liquidcrystal, and influence of the ambient light can be reduced.

However, when using, for example, a cholesteric liquid crystal forreflecting right-handed circularly polarized light, since theright-handed circularly polarized light of environmental light isreflected, sufficient improvement of the contrast cannot be realized insome cases.

Supposing a light source in natural field, almost all of the lightderives from the sunlight, and as the ambient light proceeding into theprojection system from the external light entrance, there are directlight from the sun and reflected light of which the direct light isreflected by some object. Usually, the direct light from the sun can beseparated into, to the proceeding direction, P waves (longitudinalwaves) and S waves (transversal waves). Considering reflection of thelight On a dielectric interface, while reflection is within Brewster'sangle, S waves are totally reflected, and hence the reflected lightusually contains a lot of S wave components. Specifically, if the outerfield is wet by rain, etc., a lot of S wave component is contained inthe reflected light, due to the total reflection at air-water interface.When such reflected light containing much S wave components is seen frominside of the projection system, so-called glare occurs, and visibilityis not satisfactory.

SUMMARY OF THE PRESENT INVENTION

The invention is devised in the light of the above background. The mainobject of the present invention is to present a projection systemexcellent in visibility from inside of projection system to outside, andcapable of obtaining a sharp contrast display without being influencedby the ambient light.

The present invention provides a projection system comprising: aprojection screen, containing a substrate and a polarized lightselective-reflection layer formed on the substrate and diffuse-reflectsright-handed circularly polarized light or left-handed circularlypolarized light, which displays an image by reflecting an image lightirradiated from a projector; a projector for projecting an image lighton the projection screen; and an external light screen installed at anexternal light entrance, disposed in a sequence of a linearly polarizingplate and a retardation plate from an external side, and circularlypolarizes a light in reverse direction to the circularly polarized lightdiffuse-reflected by the polarized light-selective reflection layer,wherein a transmission axis of the linearly polarizing plate is invertical direction.

Further in the invention, since the transmission axis of the linearlypolarizing plate is in vertical direction, only the light vibrating inthe vertical direction can be transmitted, and S wave component causedby total reflection at the dielectric interface can be canceled toobtain anti-glare effect and to improve visibility from inside ofprojection system to outside. Further in the invention, the polarizedlight selective reflection layer has a property of diffusing andreflecting right-handed circularly polarized light or left-handedcircularly polarized light, and the external light screen composes acircularly polarized light in reverse direction of circularly polarizedlight diffused and reflected by the polarized light selective reflectionlayer. Therefore, reflection of external light on projection screen canbe theoretically set to zero, and only image light is reflected toobtain display of sharp contrast. Moreover, in the invention, since Pwave components of light from outer field can be taken into theprojection system, a bright state can be maintained.

Preferably, in the present invention, the polarized lightselective-reflection layer has a cholesteric liquid crystal structure.The cholesteric liquid crystal structure has a spiral structure, asphysical molecular arrangement of liquid crystal molecules, in whichdirectors of the liquid crystal molecules rotate continuously in thethickness direction of the layer. On the basis of such physicalmolecular arrangement of the liquid crystal molecules, for example,polarized light separating property, which separates the circularlypolarized light in one direction from the circularly polarized light inreverse direction, can be exhibited.

Preferably, in the present invention, the external light screen is awide-band type circularly polarizing plate which circularly polarizes alight in visible light region. Since external light usually includeswavelength components of the entire visible light region, influence ofthe external light can be theoretically set to zero in the presentinvention by circularly polarizing light in the entire visible lightregion.

The present invention also provides an external light screen used in theabove-described projection system.

According to the invention, by using the external light screen, adesired circularly polarized light can be obtained. Since thetransmission axis of the linearly polarizing plate is in verticaldirection, anti-glare effect is obtained, and visibility from inside ofprojection system to outside is improved.

In the invention, since the transmission axis of the linearly polarizingplate is in vertical direction, S wave component caused by totalreflection at the dielectric interface can be canceled to obtainanti-glare effect, and to improve visibility from inside of projectionsystem to outside. Further in the inventions since the circularlypolarized light diffused and reflected by the polarized light selectivereflection layer, and the circularly polarized light composed by theexternal screen are in mutually reverse directions, reflection ofexternal light on projection screen can be theoretically set to zero toreflect only image light, so that display of sharp contrast is obtained.Moreover, in the invention, since P wave components of light from outerfield can be taken into the projection system, a bright state can bemaintained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration showing an example of the projection system ofthe present invention;

FIG. 2 is an illustration explaining the optical function of thepolarized light selective-reflection layer of the present invention;

FIG. 3 is an illustration explaining structural unevenness of thecholesteric liquid crystal structure of the polarized lightselective-reflection layer of the present invention;

FIG. 4 is a schematic sectional view showing other examples of thesubstrate in the present invention; and

FIG. 5 is a schematic sectional view showing another example of theprojection screen of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Projection system and external screen of the present invention aredescribed below.

A. Projection System

The projection system of the present invention comprises: a projectionscreen, containing a substrate and a polarized lightselective-reflection layer formed on the substrate and diffuse-reflectsright-handed circularly polarized light or left-handed circularlypolarized light, which displays an image by reflecting an image lightirradiated from a projector; a projector for projecting an image lighton the projection screen; and an external light screen installed at anexternal light entrance, disposed in a sequence of a linearly polarizingplate and a retardation plate from an external side, and circularlypolarizes a light in reverse direction to the circularly polarized lightdiffuse-reflected by the polarized light-selective reflection layer,wherein a transmission axis of the linearly polarizing plate is invertical direction.

Further in the invention, since the transmission axis of the linearlypolarizing plate is in vertical direction, only the light vibrating inthe vertical direction can be transmitted, and S wave component causedby total reflection at the dielectric interface can be canceled toobtain anti-glare effect, and to improve visibility from inside ofprojection system to outside. Further in the invention, the polarizedlight selective reflection layer has a property of diffusing andreflecting right-handed circularly polarized light or left-handedcircularly polarized light, and the external light screen composes acircularly polarized light in reverse direction of circularly polarizedlight diffused and reflected by the polarized light selective reflectionlayer. Therefore, reflection of external light on projection screen canbe theoretically set to zero, and only image light is reflected toobtain display of sharp contrast. Moreover, in the invention, since Pwave components of light from outer field can be taken into theprojection system, a bright state can be maintained.

The projection system of the present invention is described below byreferring to the drawing. For example, as shown in FIG. 1, theprojection system of the present invention comprises: a projectionscreen 1 containing a substrate and a polarized lightselective-reflection layer formed on the substrate; a projector 2 forprojecting an image light on the projection screen 1; and an externallight screen 5 installed at an external light entrance and disposed in asequence of a linearly polarizing plate 3 and a retardation plate 4 froman external side. In the invention, since the transmission axis of thelinearly polarizing plate 3 is in vertical direction, S wave componentof external light 6 does not transmit, and anti-glare effect isobtained. Besides, P ware component transmitting the linearly polarizingplate 3 transmits through the retardation plate 4 to become right-handedcircularly polarized light 7. Since the right-handed circularlypolarized light 7 is not reflected by the projection screen 1 whichdiffuses and reflects left-handed circularly polarized light 8, displayof sharp contrast is obtained.

As used herein, diffuse-reflection by the polarized lightselective-reflection layer means expansion and scattering of thereflected light (image light), reflected by the projection screen, tosuch an extent as to be recognized as an image by the observer.

Components of such projection system of the present invention areindividually described below. First, the external light screen, the mostpeculiar element of the present invention, is explained, and then theprojection screen and projector are explained.

1. External Light Screen

First, the external light screen used in the projection system of thepresent invention is explained. The external light screen used in thepresent invention is disposed in the sequence of the linearly polarizingplate and retardation plate from the external side, and this externallight screen circularly polarizes the light in reverse direction to thecircularly polarized light diffuse-reflected by the polarized lightselective-reflection layer, wherein the linearly polarizing plate is invertical direction.

In the external light screen used in the invention, since thetransmission axis of the linearly polarizing plate is in verticaldirection, only the light vibrating in the vertical direction can betransmitted, and S wave component caused by total reflection at thedielectric interface can be canceled to obtain anti-glare effect, and toimprove visibility from inside of projection system to outside. Furtherthe external light screen used in the invention composes a circularlypolarized light in reverse direction of circularly polarized lightdiffused and reflected by the polarized light selective reflection layerdescribed below. Therefore, reflection of external light on projectionscreen can be theoretically set to zero, and only image light isreflected to obtain display of sharp contrast. Moreover, since theexternal light screen takes in P wave components of light from outerfield into the projection system, a bright state can be maintained.

(1) Linearly Polarizing Plate

The linearly polarizing plate used in the present invention isdescribed. The linearly polarizing plate used in the present inventionis used for obtaining a linearly polarized light, in vertical direction,from external light.

In the invention, by using the linearly polarizing plate, only the lightvibrating in the vertical direction can be transmitted, and S wavecomponent of external light can be canceled to obtain anti-glare effect,and to improve visibility from inside of projection system to outside.Further, by transmitting the obtained linearly polarized light throughthe retardation plate described below, a desired circularly polarizedlight can be obtained.

The linearly polarizing plate used in the present invention is notparticularly limited as long as a linearly polarized light in verticaldirection can be obtained. So-called iodine based polarizing plates canbe listed for example, which are fabricated by dispersing iodine dye inpolyvinyl alcohol, aligning iodine molecules in a specified direction bydrawing process, and enclosing thereof with triacetyl cellulose layersas protective layers. Commercial iodine based polarizing plates include,for example, G1220DUN manufactured by Nitto Denko and LLC2-9118manufactured by Sanritz Co., and both can be preferably used in thepresent invention.

(2) Retardation Plate

Next the retardation plate used in the present invention is described.The retardation plate used in the present invention converts thelinearly polarized light, obtained from the above-described linearlypolarizing plate, into the circularly polarized light. And theretardation plate is used to obtain the circularly polarized light inreverse direction to the circularly polarized light diffuse-reflected bythe polarized light selective-reflection layer.

In the present invention, by using the retardation plate, a desiredcircularly polarized light can be obtained. Being in reverse directionto the circularly polarized light diffuse-reflected by the polarizedlight selective-reflection layer described below, reflection of externallight on the projection screen can be theoretically set to zero, andonly the image light is reflected, so that a display of sharp contrastcan be obtained.

The retardation plate used in the present invention is not particularlylimited as long as the linearly polarized light obtained from theabove-described linearly polarizing plate can be converted into adesired circularly polarized light, and it is preferably a wide-bandtype retardation plate which circularly polarizes the light in visiblelight region. Since the external light usually contains wavelengthcomponents across the entire visible light region, by circularlypolarizing the light across the entire visible light region, influenceby the external light can be theoretically set to zero in the presentinvention. For example, only the light in a certain visible lightregion, specifically only the highest visibility green color (around 550nm), can be circularly polarized. However, when a retardation plate,which cannot circularly polarize the light in the other visible lightregion completely, is used, a part of the light is reflected on theprojection screen so that the image contrast will be deteriorated.

To obtain a circularly polarized light, a combination of a λ/4retardation plate and a linearly polarizing plate is usually used. Tomake it a wider-band type, the following can be used: a laminated typeλ/4 retardation plate, wherein so-called a general λ/2 retardation plateof a wavelength-dispersive type and a λ/4 retardation plate are stucktogether at a certain angle, that functions as a wide-band typeretardation plate making the refractive index anisotropy larger on theshort wavelength side; and a λ/4 retardation plate of so-called reversewavelength dispersive type which makes the refractive index anisotropysmaller on the short wavelength side. Above all, a retardation film ofthe reverse wavelength dispersive type is preferably used because it islow in angle dependency to the circularly polarized degree of the light.As such retardation film of the reverse wavelength dispersive type, PureAce WR-W (Trade name: manufactured by Teijin Ltd.) can be used.

(3) External Light Screen

The external light screen used in the present invention is notparticularly limited as long as it is an external screen of which adesired circularly polarized light can be obtained. Specifically, it ispreferable to use the linearly polarizing plate mentioned above, incombination with λ/4 retardation plate as the retardation plate.Particularly, it is preferable that the external light screen is thewide-band type circularly polarizing plate, by using the wide-band typeλ/4 retardation plate as the retardation plate. Since the external lightusually contains the wavelength components across the entire visiblelight region, by circularly polarizing the light of the entire visiblelight region, influence of the external light can be theoretically setto zero in the present invention.

In the case of the above combination, the linearly polarizing plate andλ/4 retardation plate are disposed in this sequence from the externalside, and the angle of the linearly polarizing plate to the λ/4retardation plate is determined so as the light is circularly polarizeddesirably. At this time, whether to obtain right-handed or left-handedcircularly polarized light is determined by the direction of thecircularly polarized light diffuse-reflected by the projection screen.Both the circularly polarized light of right-handed and left-handed canbe obtained by adjusting the angle of λ/4 retardation plate.Incidentally, the circularly polarized light does not have anisotropy inthe light vibrating direction, and whether desired circularly polarizedlight or not is determined by relative angle of linearly polarizingplate and λ/4 retardation plate. Therefore, the direction of thetransmission axis of the linearly polarizing plate is not particularlylimited. However, in the invention, in order to cancel S waves, it isrequired to determine the transmission axis of the linearly polarizingplate in vertical direction.

The using method of the external light screen used in the presentinvention is not particularly limited. For example, by providing anadhesive layer on the surface of the linearly polarizing plate, theexternal light screen may be directly adhered to the external lightentrance such as window, or it may be installed near the external lightentrance such as window, as a blind.

2. Projection Screen

Next, the projection screen used in the projection system of the presentinvention is explained. The projection screen used in the presentinvention contains at least a substrate, and a polarized lightselective-reflection layer formed on the substrate, whichdiffuse-reflects right-handed circularly polarized light or left-handedcircularly polarized light, and is designed to display an image byreflecting the image light irradiated from the projector. The projectionscreen used in the present invention has, for example as shown in FIG.2, a substrate 10, and a polarized light selective-reflection layer 11formed on the substrate 10.

In the present invention, since the polarized light selective-reflectionlayer diffuse-reflects the right-handed circularly polarized light orleft-handed circularly polarized light selectively, for example, whenthe polarized light selective-reflection layer, which diffuse-reflectsthe right-handed circularly polarized light, is used, the left-handedcircularly polarized light is not reflected by the circularly polarizedlight separation function, and influence of the external light can bereduced. Further when used in combination with the external light screenmentioned above, reflection of the external light on projection screencan be theoretically set to zero.

(1) Polarized Light Selective-Reflection Layer

The polarized light selective-reflection layer used in the presentinvention is described. The polarized light selective-reflection layerused in the present invention diffuse-reflects the right-handedcircularly polarized light or left-handed circularly polarized light.

In the present invention, by providing such polarized lightselective-reflection layer, the right-handed circularly polarized lightor left-handed circularly polarized light projected from the projectoris diffuse-reflected, and a projection screen, which serves the purposeof the present invention, can be obtained.

In the polarized light selective-reflection layer, non-polarized lightentering along the spiral axis is separated into two polarized lights(right-handed circularly polarized light and left-handed circularlypolarized light), and one is transmitted and the other is reflected.This phenomenon is known as circularly dichroism. For example, when aspiral winding direction in spiral structure of liquid crystal moleculesis properly selected, circularly polarized light element, having thesame optical rotatory direction as this spiral winding direction, isselectively reflected.

In this case, maximum optical rotatory scatter occurs at wavelength λoin the following formula (1):λo=nav·p  (1)wherein p is spiral pitch length in spiral structure of liquid crystalmolecules (length per pitch of molecular spiral of liquid crystalmolecules), and nav is average refractive index in an orthogonal planeto the spiral axis.

At this time, wavelength band width Δλ of reflected light is expressedin the following formula (2), wherein Δn is birefringence value:Δλ=Δn·p  (2)

That is, as shown in FIG. 2 for example, the non-polarized lightentering from the observer side of the projection screen (right-handedcircularly polarized light 12R and left-handed circularly polarizedlight 12L within the selective-reflected wavelength region, right-handedcircularly polarized light 13R and left-handed circularly polarizedlight 13L outside of the selective-reflected wavelength region) isseparated according to the above-described polarized light separatingproperty, so that one circularly polarized light element (for example,right-handed circularly polarized light 12R within theselective-reflected wavelength region), which belongs to the range(selective-reflected wavelength region) of the wavelength band width Δλthat is centered on selective-reflection central wavelength λo, isreflected as reflected light 14, and other lights (for example,left-handed circularly polarized light 12L within theselective-reflected wavelength region, right-handed circularly polarizedlight 13R and left-handed circularly polarized light 13L outside of theselective-reflected wavelength region) are transmitted.

Such polarized light selective-reflection layer is not particularlylimited as long as the above-described right-handed circularly polarizedlight or left-handed circularly polarized light can bediffuse-reflected. Specifically, the following cases, etc. can belisted: a case of comprising a polarized light reflection layer forreflecting a specific circularly polarized light, and a diffusionelement for diffusing the light reflected by the polarized lightreflection layer; or a case wherein the polarized lightselective-reflection layer itself has diffusing properly. The polarizedlight selective-reflection layer used in the present invention isspecifically described below by dividing into two cases.

(i) A Case of Comprising a Polarized Light Reflection Layer forReflecting a Specific Circularly Polarized Light, and a DiffusionElement for Diffusing the Light Reflected by the Polarized LightReflection Layer

The polarized light selective-reflection layer in this case is explainedby dividing into polarized light reflection layer and diffusion element.

(a) Polarized Light Reflection Layer

The polarized light reflection layer used in the present inventionreflects a specific circularly polarized light. Specific examples are:ones which reflect light by mirror reflection; and ones having acholesteric liquid crystal structure of planar alignment state. By usingsuch polarized light reflection layer, polarized light separatingproperty can be obtained. Therefore, only the specific circularlypolarized light, out of the light projected by the projector, can bereflected. Further, by the diffusion element described below, the lightreflected by the polarized light reflection layer can be diffused, sothat it can be used in a bright environment, and a projection screenhigh in brightness and excellent in visibility can be obtained.

(b) Diffusion Element

Next the diffusion element is explained. The diffusion element in thepresent invention diffuses the light reflected by the polarized lightreflection layer. Such diffusion element is provided on the observerside of the above-described polarized light reflection layer fordiffusing the light reflected by the polarized light reflection layerand emitting to the observer side of the projection screen.

Specifically, the diffusion element can be composed of bulk diffusionmember, surface diffusion member, hologram diffusion layer, or anarbitrary combination of these diffusion layers. Specifically, the bulkdiffusion member includes particles disposed in a transparent medium.The surface diffusion member includes structural plane, fine structuralplane, roughened plane, etc. Diffusion achieved by the diffusion membermay be either random or regular, or it may be partly regular.

(ii) A Case Wherein the Polarized Light Selective-Reflection LayerItself has Diffusing Properly

In this case, the polarized light selective-reflection layer is notparticularly as long as the layer itself has diffusing properly.Specifically, ones having a cholesteric liquid crystal structure,wherein the cholesteric liquid crystal structure is formed structurallyuneven so that the projected light is separated in polarization anddiffused, can be used. Such polarized light selective-reflection layeris explained below.

Since such polarized light selective-reflection layer has a cholestericliquid crystal structure, the physical molecular arrangement of liquidcrystal molecules is a spiral structure in which directors of liquidcrystal molecules rotate continuously in the layer-thickness direction.And on the basis of such physical molecular arrangement of the liquidcrystal molecules, the layer has polarized light separating property ofseparating the light into, for example, the circularly polarized lightin one direction and circularly polarized light in reverse directionthereof. That is, in such polarized light selective-reflection layer,non-polarized light entering along the spiral axis is separated into twopolarized lights (right-handed circularly polarized light andleft-handed circularly polarized light), and one is transmitted and theother is reflected. This phenomenon is known as circular dichroism. Whenspiral winding direction in the spiral structure of the liquid crystalmolecules is properly selected, the circularly polarized light elementhaving the same optical rotatory direction as this spiral windingdirection is selectively reflected.

Therefore, when the polarized light selective-reflection layer havingcholesteric liquid crystal structure is used, by making the polarizedlight selective-reflection layer a layer, which reflects the specificwavelength of the same polarized light as the light emitted from theprojector, the projected light can be reflected efficiently, and aprojection screen of high brightness can be obtained. By transmittingexternal light or the like through the external light screen mentionedabove, a circularly polarized light in reverse direction of circularlypolarized light diffused and reflected by the polarized light selectivereflection layer is obtained. Therefore, reflection of external light onprojection screen can be theoretically set to zero to obtain display ofsharp contrast. Moreover, P wave components of light from outer fieldcan be taken into the projection system, and a bright state can bemaintained.

The polarized light selective-reflection layer having such cholestericliquid crystal structure preferably has the cholesteric liquid crystalstructure formed structurally uneven so that the projected light may beseparated in polarization and diffused.

As used herein, to form structural uneven cholesteric liquid crystalstructure means that direction of alignment of each liquid crystal phaseis not uniform in one direction, but is disturbed, when the polarizedlight selective-reflection layer is aligned. Specifically, the followingsituations, etc. can be listed: as shown in FIG. 3, a situation in whichthe direction of spiral axis L of spiral structural region 30 includedin cholesteric liquid crystal structure of polarized lightselective-reflection layer 11 is scattered; although not shown in thefigure, a situation in which at least a part of the nematic layer plane(a plane in which the directors of the liquid crystal molecules are samein XY direction) is not parallel to the plane of polarized lightselective-reflection layer (a situation in which, when a sectional TEMphotograph of stain-treated cholesteric liquid crystal structuralmembrane is taken, one continuous curve of a layer appearing in contrastpattern is not parallel to the substrate surface); or a situation inwhich fine particles of cholesteric liquid crystal are dispersed aspigment. In any case, since the cholesteric liquid crystal structure isformed structurally uneven so that the projected light is separated inpolarization and diffused, the image light projected from the projectoris diffuse-reflected, not mirror-reflected, the image can be recognizedeasily. Since the polarized light selective-reflection layer at thistime diffuses the selectively reflected light by its structuralunevenness, a specific circularly polarized light can be diffusinglyreflected, while other light can be transmitted without being diffused.Accordingly, there is no problem of depolarization in the image light orthe ambient light passing through the polarized lightselective-reflection layer, and the visibility of the image is improvedwhile maintaining the polarized light separating function of thepolarized light selective-reflection layer.

For example, method of forming the cholesteric liquid crystal structurestructurally uneven is not particularly limited. For example, thefollowing methods, etc. can be listed: a method in which thebelow-described substrate having no alignment in a specific direction isused; a method in which an amount of a photopolymerization initiator ora leveling agent used generally in formation of the polarized lightselective-reflection layer is adjusted; a method in which non-liquidcrystalline polymerizable compound is added in the polarized lightselective-reflection layer; and a method in which liquid crystallinefine particles are contained. These methods may be arbitrarily selected,or these methods may be combined properly.

In the polarized light selective-reflection layer having a cholestericliquid crystal structure, wherein this cholesteric liquid crystalstructure is formed structurally uneven, a wavelength region, of whichthe reflecting strength thereof is one half or more of the maximumreflecting strength of the polarized light selective-reflection layer,is preferably only a part of visible light region (for example,wavelength region of 400 nm to 700 nm). As a result, light of specificwavelength in visible light region can be selectively reflected. Asmentioned above, since the cholesteric liquid crystal reflects only thelight of specific wavelength strongly, light in wavelength other thanthis specific wavelength is almost entirely absorbed into the substrateand the like. The wavelength region, which is reflected by thecholesteric liquid crystal for constituting the polarized lightselective-reflection layer, is determined by the length of spiral pitchof the cholesteric liquid crystal.

The polarized light selective-reflection layer may be composed of onetype of spiral pitch length, as long as the light of the wavelengthirradiated, from the light source of projector, etc. can be reflected.However, for example, when the wavelength region of red (R) and green(G) is included in the wavelength band width of selective-reflectionwavelength region of one spiral pitch length, it is preferred to havespiral pitch length of these wavelengths and spiral pitch length of blue(B), and particularly, it is preferred to have spiral pitch lengths ofwavelengths of each red (R), blue (B) and green (G). This is because thelight emitted from the projector is composed of red (R), blue (B) andgreen (G), and color display is realized by these three primary colors.

In the present invention, although it depends on the types of theprojector, the specific wavelength to be selectively reflected ispreferably the wavelength of 430 nm to 460 nm of blue (B), 540 nm to 570nm of green (G), and 580 nm to 620 nm of red (R). As a result, colordisplay is possible even if there is a difference in wavelength due todesign of the apparatus or types of the light source, and a projectionscreen capable of expressing while color favorably can be realized.

The polarized light selective-reflection layer having such plural spiralpitch lengths can be composed by laminating layers having thecholesteric liquid crystal structure having each spiral pitch length.

The polarized light selective-reflection layer having cholesteric liquidcrystal structure, wherein the cholesteric liquid crystal structure isformed structurally uneven (each layer, when the polarized lightselective-reflection layer is composed of plural layers), preferably hasa film thickness which reflects specified polarized light by 100%.Reflectivity of the polarized light selective-reflection layer for thepolarized light depends on the film thickness of the polarized lightselective-reflection layer. Therefore, when the reflectivity for thespecific circularly polarized light (for example, right-handedcircularly polarized light) selectively reflected is less than 100%, theimage light cannot be reflected efficiently. To achieve 100% ofreflectivity, usually, 4 pitches to 8 pitches are preferred.Specifically, it is usually 1 μm to 10 μm, although it depends on thetypes of material of polarized light selective-reflection layer orwavelength of circularly polarized light. When the thickness is thinnerthan the above-described thickness, the reflectivity is lowered, and itis difficult to reproduce the image projected on the projection screenat high brightness. On the other hand, when the thickness is thickerthan the above-described thickness, it is difficult to control thecholesteric liquid crystal structure, or unevenness may occur.

In the case in which the polarized light selective-reflection layer hasthe cholesteric liquid crystal structure, wherein the cholesteric liquidcrystal structure is formed structural unevenly so that the projectedlight is separated in polarization and diffused, the material for suchpolarized light selective-reflection layer includes chiral nematicliquid crystal and cholesteric liquid crystal, and a material havingcholesteric regularity may be used without limitation. Particularly, apolymerizable liquid crystal material having polymerizable functionalgroups on both ends of molecule is preferable. As a result, aftercuring, an optically stable projection screen can be obtained. If thepolymerizable liquid crystal material has nematic regularity or smecticregularity, a polymerizable chiral agent may be used. Materials used inpolarized light selective-reflection layer and forming method ofpolarized light selective-reflection layer in the present invention aredescribed below.

(a) Polymerizable Liquid Crystal Material

An example of such polymerizable liquid crystal material havingpolymerizable functional group is, for example, a compound (I) shown inthe following general formula (1). As the compound (I), mixture of twokinds of compounds included in the general formula (1) can be used.Further, the polymerizable liquid crystal material may be composed ofthe above-described compound (I) and the compound (II) shown in thefollowing general formulas (2) to (12).

As the compound (I), mixture of two kinds of compounds included in thegeneral formula (1) can be used.

wherein x is an integer from 2 to S.

In the general formula (1) showing compound (I), R¹ and R² each shows ahydrogen or a methyl group, but both R¹ and R² are preferably hydrogenbecause of the width of temperature range showing liquid crystal phase.X can be any one of hydrogen, chlorine, bromine, iodine, or alkyl group,methoxy group, cyano group, or nitro group with 1 to 4 carbon atoms.However, chlorine or methyl group is preferable. Chain lengths (a) and(b) of alkylene groups, which are spacers between the (meth) acryloyloxygroup and the aromatic ring on both ends of molecular chain of compound(I), can be individually arbitrary integers in a range of 2 to 12,preferably in a range of 4 to 10, more preferably in a range of 6 to 9.Compound of the general formula (1), in which a=b=0, lacks in stability,is likely to be hydrolyzed, and the compound itself is high incrystallinity. Compound of the general formula (1), in which both (a)and (b) are 13 or more, is low in isotropic transition temperature (TI).For this reason, these compounds are not preferable because thetemperature range showing liquid crystal property of these compounds isnarrow.

In the above example, polymerizable liquid crystal monomer is shown.However, polymerizable liquid crystal oligomer, polymerizable liquidcrystal polymer, etc. can be also used in the present invention. Forsuch polymerizable liquid crystal oligomer or polymerizable liquidcrystal polymer, known materials can be properly selected and used.

(b) Chiral Agent

In the present invention, chiral nematic liquid crystal havingcholesteric regularity, prepared by adding chiral agent to nematicliquid crystal, is preferably used.

The chiral agent used in the present invention means a low molecularcompound having an optically active position whose molecular weight is1500 or less. The chiral agent is mainly used for the purpose ofinducing spiral pitch in positive uniaxial nematic regularity expressedby polymerizable liquid crystal material, for example, shown in compound(I) or compound (II) used as required. As long as this purpose isfulfilled, the types of the low molecular compound as chiral agent shownbelow is not particularly limited, if: it has solubility in a solutionstate or in a melting state with, for example, compound (I) or mixtureof compound (I) and compound (II); and a desired spiral pitch can beinduced without spoiling the crystallinity of polymerizable liquidcrystal material having nematic regularity. However, a molecular havingpolymerizable functional groups on both ends is preferable from theviewpoint of obtaining optical element of excellent heat resistance. Itis essential, for the chiral agent used to induce spiral pitch for theliquid crystal, to have some chirality, at least in the molecule.Therefore, the chiral agent usable in the present invention are, forexample, a compound having one or two or more asymmetric carbon, acompound having asymmetrical point on hetero atom such as chiral amineor chiral sulfoxide, or a compound having axial asymmetry such ascumulene or binaphthol. Specifically, commercial product of chiralnematic liquid crystal, for example, S-811 manufactured by Merck can belisted.

However, depending on the properties of the selected chiral agent, thenematic regularity formed by the polymerizable liquid crystal material,such as compound (I) and mixture of compound (I) and compound (II), maybe destroyed, the alignment property may be lowered, or if the compoundis non-polymerizable, curing property of the liquid crystal compositionmay be lowered, or reliability of cured film may be deteriorated.Further, a heavy use of the chiral agent having optically activeposition leads to increase of cost of the composition. Therefore, whenmanufacturing circularly polarized light controlling optical elementhaving cholesteric regularity of short pitch, as the chiral agent havingan optically active position to be contained in the polymerizable liquidcrystal material used in the present invention, it is preferable toselect a chiral agent having large effect of inducing spiral pitch.Specifically, it is preferable to use a low molecular compound (III),having axial asymmetry in the molecule, as expressed in the followinggeneral formula (13), (14) or (15).

In the general formula (13) or (14) expressing chiral agent (III), R⁴shows hydrogen or methyl group. Y is any optional one of formulae (i) to(xxiv) shown above, and among the above, any one of formulae (i), (ii),(iii), (v) and (vii) is preferable. Chain lengths (d) and (e) ofalkylene group are individually arbitrary integers in a range of 2 to12, and preferably in a range of 4 to 10, and more preferably in a rangeof 6 to 9. Compound of the general formula (13) or (14), in which thevalue of (d) or (e) is 0 or 1, lacks instability, and is likely to behyrdolyzed, and is high in crystallinity. On the other hand, thecompounds, in which the value of (d) or (e) is 13 or more, is low inmelting point (Tm). Hence, these compounds lower solubility withcompound (I) or mixture of compound (I) and compound (II) showingcrystallinity, and phase separation may occur depending on theconcentration.

The optimum amount of the chiral agent blended in the polymerizableliquid crystal material of the present invention is determined inconsideration of the spiral pitch inducing capability or cholestericproperty of the finally obtained circularly polarized light controllingoptical element. Specifically, although significantly variable dependingon the polymerizable liquid crystal material to be used, it is selectedin a rang of 0.01 to 60 parts by weight, preferably 0.1 to 40 parts byweight, more preferably 0.5 to 30 parts by weight, and most preferably 1to 20 parts by weight, per 100 parts by weight of the total amount ofthe polymerizable liquid crystal material. If the blending amount issmaller than the above range, sufficient cholesteric property may not begiven to the polymerizable liquid crystal material, or if exceeding theabove range, alignment of molecules is inhibited, and may be adverselyinfluenced when curing by active irradiation.

In the present invention, the chiral agent is not always required to bepolymerizable. However, considering the thermal stability of theobtained polarized light selective-reflection layer, it is preferable touse the polymerizable chiral agent, which is polymerized with theabove-described polymerizable liquid crystal material and is able tosolidify the cholesteric regularity.

(c) Others

Besides the polymerizable liquid crystal material and chiral agent, thepolarized light selective-reflection layer in this case may includeother materials generally used in a polarized light selective-reflectionlayer, as required, such as photopolymerization initiator, intensifyingagent, leveling agent, etc.

Photopolymerization initiator used in the present invention includes,for example, benzyl (or called bibenzoyl), benzoin isobutyl ether,benzoin isopropyl ether, benzophenone, benzoyl benzoate, methylbenzoylbenzoate, 4-benzoyl-4′-methyl diphenyl sulfide, benzyl methylketal, dimethyl amino methyl benzoate, 2-n-butoxy ethyl-4-dimethyl aminobenzoate, isoamyl p-dimethyl aminobenzoate,3,3′-dimethyl-4-methoxybenzophenone, methylobenzoyl formate,2-methyl-1-(4-(methylthio) phenyl)-2-morpholinopropane-1-on,2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butane-1-on,1-(4-dodecylphenyl)-2-hydroxy-2-methylpropane-1-on, 1-hydroxycyclohexylphenyl ketone, 2-hydroxy-2-methyl-1-phenylpropane-1-on,1-(4-isopropylphenyl)-2-hydroxy-2-methylpropane-1-on,2-chlorothioxanthone, 2,4-diethyl thioxanthone, 2,4-diisopropylthioxanthone, 2,4-dimethyl thioxanthone, isopropyl thioxanthone,1-chloro-4-propoxy thioxanthone, etc. Aside from photopolymerizationinitiator, an intensifier may be added, in a range not to spoil thepurpose of the present invention.

The content of photopolymerization initiator used in the presentinvention is in a range of 0.01 to 20 wt %, preferably 0.1 to 10 wt %,or more preferably 0.5 to 5 wt %.

In the present invention, as described above, the cholesteric liquidcrystal structure of the polarized light selective-reflection layer maybe formed structurally uneven, so that the projected light is separatedin polarization and diffused, by adjusting the contents of thesematerials. For example, by increasing the content of photopolymerizationinitiator, molecular chain of the cholesteric liquid crystal isshortened, and alignment of the cholesteric liquid crystal surface maybe disturbed. At this time, the photopolymerization initiator afterreaction plays the role of impurity to disturb the alignment of thecholesteric liquid crystal in the cholesteric liquid crystal.

Moreover, by adding a non-liquid crystalline polymerizable compound nothaving liquid crystal alignment, the alignment of the cholesteric liquidcrystal is disturbed, and the structure can be formed uneven. By addingliquid crystalline fine particles, alignment of the cholesteric liquidcrystal may be disturbed. In the present invention, these methods may beproperly combined, and the types and contents of these additives may beproperly selected depending on the purpose.

(2) Substrate

The substrate used in the projection system used in the presentinvention is explained. The substrate used in the projection system ofthe present invention is not particularly limited as long as thepolarized light selective-reflection layer can be formed. In the presentinvention, it is preferable to use a substrate which absorbs the lightof wavelength in visible light region. Specifically, the substratepreferably absorbs light in a range of 400 nm to 700 nm. Accordingly,for example, when the polarized light selective-reflection layer isformed from the liquid crystalline composition having cholestericregularity, reflection of the circularly polarized light, which ispolarized to the reverse direction to the circularly polarized light ofthe cholesteric liquid crystal, can be prevented, and a projectionscreen high in brightness can be obtained.

Substrate absorbing wavelength in visible light region includes, forexample as shown in FIG. 4A, a plastic film 40 of which black pigment iskneaded therein. Moreover, as shown in FIG. 4B, a light absorbing layer42 may be formed on a transparent plastic film 41. This light absorbinglayer 42 may be formed on the polarized light selective-reflectionlayer-formed side, or as shown in FIG. 4C, the layer may be formed onthe opposite side.

In the present invention, for example, when the polarized lightselective-reflection layer has the cholesteric liquid crystal structure,wherein the cholesteric liquid crystal structure is formed structuraluneven so that the projected light is separated in polarization anddiffused, the substrate may be made of a material with few surfacealignment, in order to form the cholesteric liquid crystal structurallyuneven. Such material with few surface alignment includes, for example,non-drawn plastic film, or non-rubbed material. In order to obtain goodregularity, the polarized light selective-reflection layer in such caseis usually formed on a drawn or rubbed plastic film. However, in thepresent invention, by forming the polarized light selective-reflectionlayer on the substrate not drawing or rubbing processed, the liquidcrystals on the substrate surface are not aligned regularly, and thealignment of the cholesteric liquid crystal structure can be disturbedso that the light projected from the projector may be separated inpolarization and diffused.

Materials for substrate are not particularly limited. For example,thermoplastic polymers film, metal, paper, glass, etc. can be listed.Examples of plastic film are films made of: polycarbonate based polymer;polyester based polymer such as polyallylate and polyethyleneterephthalate; polyimide based polymer; polysulfone based polymer;polyether sulfone based polymer; polystyrene based polymer; polyolefinbased polymer such as polyethylene and polypropylenep; polyvinyl alcoholbased polymer; cellulose acetate based polymer; polyvinyl chloride basedpolymer; polymethyl methacrylate based polymer; etc.

Film thickness of the substrate used in the present invention may beproperly selected depending on the application and type of theprojection screen. For example, when the projection screen is used inwinding-up type, it is usually 15 μm to 300 μm, specifically 25 μm to100 μm. When the projection screen is not used in the winding-up type,but used in panel type or the like, wherein flexibility is not required,the thickness of the substrate is not particularly limited.

The substrate used in the present invention may be surface-treated suchas corona treatment and UV cleaning, in order to enhance adhesion withthe polarized light selective-reflection layer.

Further, a plastic film provided with a tacky layer may be used. Forexample, PET film with tacky layer A4100 (trade name of Toyobo), tackymaterials AC-X, AC-L, AC-W (trade names of Panak), etc. can be used.

(3) Projection Screen

The projection screen of the present invention is not particularlylimited as long as the above-described polarized lightselective-reflection layer is formed on the above-described substrate.For example, as shown in FIG. 5, it is enough if an adhesion improvinglayer 51 is formed on the substrate 10, and the polarized lightselective-reflection layer 11 is formed on this adhesion improving layer51. Further, an ultraviolet absorbing layer 52 may be formed on thepolarized light selective-reflection layer 11. As mentioned above, thepolarized light selective-reflection layer is not limited to one layer,but may comprise, as shown in FIG. 5 for example, red polarized lightselective-reflection layer (11R), green polarized lightselective-reflection layer (1G), blue polarized lightselective-reflection layer (11B), etc., and further, layers of othercolors may be also provided. Still more, as mentioned above, theultraviolet absorbing layer 52 may have only ultraviolet absorbingfunction, but may have, in addition to ultraviolet absorbing function,also reflection preventive function, antiglare function, hard coatingfunction, or other functions. Depending on the embodiment of theultraviolet absorbing layer, for example, a reflection preventive layer,an antiglare layer, a hard coating layer, etc. may be formed separately.

In the present invention, it is preferred to form the adhesion improvinglayer. This adhesion improving layer is provided in order to enhance theadhesion between the substrate and polarized light selective-reflectionlayer. In the adhesion improving layer, type, material, etc. is notparticularly limited. For example, acrylic based material or epoxy basedmaterial may be used.

In the present invention, illumination of the room in which theprojection screen is used, is preferably the circularly polarized lightopposite to the circularly polarized light reflected by the projectionscreen. As a result, even if the illuminating light incident on theprojection screen, the projection screen does not reflect this light,but absorbs the light, so that high brightness can be obtained even in abright environment. At this time, the illumination can be controlled byusing reflecting type circular polarizer and the like, which usesabsorbing type circular polarizer, circularly polarized light separationlayer, linearly polarized light separation layer.

(4) Manufacturing Method of Projection Screen

Manufacturing method of projection screen in the present invention isexplained. Manufacturing method of the projection screen in the presentinvention comprises, after adjusting the above-described substrate, apolarized light selective-reflection layer forming step of forming thepolarized light selective-reflection layer by coating a composition, amixture of materials for forming polarized light selective-reflectionlayer, on this substrate, thereby forming the polarized light selectivereflection layer.

Manufacturing method of projection screen in the present invention isnot particularly limited as long as the projection screen serving thepurpose of the present invention can be obtained. This is to explain, asa specific example, a manufacturing method of projection screen havingthe cholesteric liquid crystal structure, wherein the cholesteric liquidcrystal structure is formed structurally uneven so that the projectedlight is separated in polarization and diffused.

As a method of coating the composition on the substrate, thecomposition, a mixture of each material, may be coated as it is.However, from the viewpoint of adjustment of viscosity or alignment, itis preferred that the composition is dissolved in organic solvent. Thesolvent is not particularly limited as long as the substrate is noteroded. For examples, acetone, acetic acid-3-methoxy butyl, diglyme,cyclohexanone, tetrahydrofuran, toluene, xylene, chlorobenzene,methylene chloride, methyl ethyl keton, etc can be used. In this case,the composition is usually diluted to 5 wt % to 50 wt %, more preferably10 wt % to 30 wt %.

The composition may be coated by any general method. For example, thecoating can be carried out by roll coating method, gravure coatingmethod, bar coating method, slide coating method, die coating method,slit coating method, immersion method, etc. When the substrate is aplastic film, the composition may be coated by roll-to-roll film coatingmethod.

Successively, the composition is maintained at the specifiedtemperature, at which the cholesteric liquid crystal structure isexhibited, and the composition is aligned. The cholesteric liquidcrystal structure of the polarized light selective-reflection layerobtained finally is not in a planar alignment state, but is in disturbedalignment state with structural unevenness. However, also in this case,an alignment treatment is required. That is, an alignment treatment, inwhich the directors of the liquid crystal molecules of the cholestericliquid crystal structure is aligned in a specific direction on thesubstrate, is not required. However, an alignment treatment, in which aplurality of spiral structure regions is formed in the cholestericliquid crystal structure, is required.

The alignment treatment can be carried out by maintaining thecomposition at the specified temperature at which the cholesteric liquidcrystal structure is exhibited. By this, the cholesteric liquid crystalexhibits a liquid crystal phase, and by the self-integration function ofthe liquid crystal molecule itself, spiral structure, in which thedirectors of the liquid crystal molecules are rotated continuously inthickness direction of the layer, is formed. The cholesteric liquidcrystal structure expressed in such liquid crystal phase state cansolidify the cholesteric liquid crystal by the following means.

When the solvent is contained in the liquid crystalline compositioncoated on the substrate, such alignment treatment is usually carried outtogether with a drying treatment for removing the solvent. To remove thesolvent, drying temperature is preferably of 40 to 120° C., morepreferably 60 to 100′. The drying time (heating time) is notparticularly specified as long as the cholesteric liquid crystalstructure is expressed and the solvent is removed substantially, and forexample, it is preferably 15 to 600 seconds, more preferably 30 to 180seconds. When the alignment is found out to be insufficient afterdrying, heating time may be extended properly. When using a reducedpressure drying means in such drying treatment, another heatingtreatment is preferably carried out for the alignment treatment.

Next, the liquid crystal molecules in the polarized lightselective-reflection layer aligned in the above-described alignmenttreatment are subjected to a solidification process, in which thecholesteric liquid crystal structure is solidified. Thereby, thecholesteric liquid crystal structure expressed in the liquid phase stateis fixed.

As the methods used in the solidifying process, the following can belisted: (i) a method in which the solvent in the liquid crystallinecomposition is dried; (ii) a method in which the liquid crystalmolecules in the liquid crystalline composition is polymerized byheating; (iii) a method in which the liquid crystal molecules in theliquid crystalline composition is polymerized by irradiating radioactiveray; and

(iv) a method in which these methods are combined.

Among the above, the method of (i) is a method suited when using liquidcrystal polymer as the liquid crystal material showing nematicregularity contained in the liquid crystalline composition, which is amaterial for the polarized light selective-reflection layer. In thismethod, the iquid crystal polymer is coated on the substrate in a stateof being dissolved in a solvent such as organic solvent. In this case,only by removing the solvent by drying treatment, a solidified polarizedlight selective-reflection layer having cholesteric regularity isformed. As for the type of solvent and drying condition, those explainedin the above-described coating treatment or alignment treatment can beused.

The method of (ii) is a method in which the polarized lightselective-reflection layer is cured by polymerizing the liquid crystalmolecules in the liquid crystalline composition by heating. In thismethod, since the bonding condition of the liquid crystal molecules ischanged by heating (calcinating) temperature, if the temperature withinthe plane of polarized light selective-reflection layer is uneven atheating, unevenness occurs in the properties such as film hardness oroptical propery. To maintain the distribution of film hardness within±10%, it is preferable to maintain the distribution of heatingtemperature within +5%, more preferably within +2%.

Heating method of the polarized light selective-reflection layer formedon the substrate is not particularly specified as long as uniformity ofheating temperature can be obtained. The layer may be held tightly on ahot plate, or the layer may be held in parallel to a hot plate, with aslight air layer provided therebetween. Moreover, the layer may beplaced inside an apparatus for heating a specific space entirely, suchas oven, or the layer may be passed through the apparatus. When using afilm coater or the like, it is preferred to heat for a sufficient timeby extending the heating zone.

As for the heating temperature, temperature of 100° C. or higher isusually required. In consideration of heat resistance of the substrate,the temperature up to 150° C. is preferable. However, it is alsopossible to heat over 150° C. when using a heat resistant film assubstrate.

The method of (iii) is a method in which the polarized lightselective-reflection layer is cured by photopolymerizing the liquidcrystal molecules in the liquid crystalline composition by irradiatingradioactive ray. In this method, proper radioactive ray, such aselectron ray and ultraviolet ray, may be used depending on theconditions. Usually, ultraviolet ray is preferably used because of thesimpleness of the apparatus, and its wavelength is 250 to 400 nm. Whenusing ultraviolet ray, photopolymerization initiator is preferablyadded, as mentioned above, to the liquid crystalline composition.Incidentally, content of the photopolymerization initiator to be addedto the liquid crystalline composition is in a range of 0.1 to 20 wt %,preferably 0.1 to 10 wt %, more preferably 0.5 to 5 wt %.

By carrying out this series of treatment (coating treatment, alignmenttreatment, and solidifying treatment), a projection screen having asingle layer of the polarized light selective-reflection layer can bemanufactured. By repeating this series of treatment, a projection screenhaving a plurality of layers of the polarized light selective-reflectionlayer can be manufactured. When the polarized light selective-reflectionlayer is further coated on the polarized light selective-reflectionlayer having light diffusion property, since the alignment state of thelower layer is continued, an alignment controlling layer is notparticularly needed. However, other layers such as tacky layer may beformed.

The projection screen used in the present invention may be provided withan ultraviolet absorbing layer, as required. The ultraviolet absorbinglayer may be formed, for example, by coating an ultraviolet absorbinglayer forming coating solution on the projector side of the polarizedlight selective-reflection layer. Methods of coating include,specifically, roll coating method, gravure coating method, bar coatingmethod, slide coating method, die coating method, slit coating method,immersing method, etc. When the substrate is a plastic film, it may becoated by roll-to-roll film coating method.

In the projection screen used in the present invention, by forming anadhesion improving layer, a reflection preventive layer, an anti-glarelayer, a hard coat layer, etc. as required, the projection screen havingthereof can be manufactured.

3. Projector

The projector used in the projection system of the present invention isexplained. The projector used in the present invention is notparticularly limited as long as it can display an image on theprojection screen by contrast of light. For example, a cinematograph,which forms an image by disposing a film and the like in front of thelight source, may be used. In the present invention, among the above, itis preferred to use projectors of spontaneous light type such as CRTsystem, liquid crystal system, or light bulb type such as DLP system.When circularly polarizing the emitted light, for example in the case ofprojector of liquid crystal system, by transmitting through aretardation plate which converts the emitted linearly polarized lightinto circularly polarized light, light can be converted into circularlypolarized light with almost no loss in quantity of light. As theretardation plate used in this case, a λ/4 retardation plate ispreferred. It is preferable to use ones of 137.5 nm, conforming to 550nm, the highest visibility. Further, to be applicable to all wavelengthof emitted RGB, a wide-band type λ/4 retardation plate is particularlypreferable. As the wide-band type λ/4 retardation plate, the same onesas the retardation plate mentioned in “1. External light screen, (2)Retardation plate” above can be preferably used as well. The retardationplate may be either assembled in the projector, or externally attachedat the exit.

In the projector of CRT system or DLP system, since the emitted light isnot controlled in polarization, it is preferable to polarize the lightlinearly via the optical element, and to dispose the retardation plate.In this case, the quantity of light of the projector is reduced to half,but the contrast improving effect can be obtained.

B. External Light Screen

External light screen of the present invention is described. Externallight screen of the present invention is used in the projection systemmentioned above. By using the external light screen, a desiredcircularly polarized light can be obtained. Since the transmission axisof the linearly polarizing plate is in vertical direction, anti-glareeffect is obtained, and visibility from inside of projection system tooutside is improved.

The external light screen of the present invention includes the linearlypolarizing plate and retardation plate. These members are as same asthose explained in “A. Projection system” above, and explanation isomitted herein.

The embodiment of the external light screen of the present invention isnot particularly limited as long as the desired effects can be obtainedin the projection system. For example, it may be a pasting type externallight screen which is pasted directly to the external light entrance byproviding an adhesive layer on the surface of the linearly polarizingplate positioned at the outer side of the external light screen. Insteadof pasting directly on the external light entrance as described above,the external light screen may be used as a blind or partition near theexternal light entrance. The pasting type external light screen ispreferably used, for example, in conference room and the like in whichthe projection system is permanently installed. The external lightscreen used as the blind or partition can be preferably used in, forexample, household projection system and the like since desired effectscan be obtained only when using the projection system by installing themon the window for taking in external light sufficiently usually. Sincethe linearly polarizing plate used in the external light screen of thepresent invention is usually manufactured by drawing treatment, theexternal light screen for long external light entrance can bepreferentially manufactured industrially.

The present invention is not limited to the embodiments described above.The embodiments described above are merely illustrative, and anyembodiments having substantially the same constitution and exhibitingthe same function and effect as the technical ideas described in claimsof the present invention is included in the technical scope of thepresent invention.

1. A projection system comprising: a projection screen, containing asubstrate and a polarized light selective-reflection layer formed on thesubstrate and diffuse-reflects right-handed circularly polarized lightor left-handed circularly polarized light, which displays an image byreflecting an image light irradiated from a projector; a projector forprojecting an image light on the projection screen; and an externallight screen installed at an external light entrance, disposed in asequence of a linearly polarizing plate and a retardation plate from anexternal side, and circularly polarizes a light in reverse direction tothe circularly polarized light diffuse-reflected by the polarizedlight-selective reflection layer, wherein a transmission axis of thelinearly polarizing plate is in vertical direction.
 2. The projectionsystem according to claim 1, wherein the polarized lightselective-reflection layer has a cholesteric liquid crystal structure.3. The projection system according to claim 1, wherein the externallight screen is a wide-band type circularly polarizing plate whichcircularly polarizes a light in visible light region.
 4. The projectionsystem according to claim 2, wherein the external light screen is awide-band type circularly polarizing plate which circularly polarizes alight in visible light region.
 5. An external light screen used in theprojection system according to claim
 1. 6. An external light screen usedin the projection system according to claim
 2. 7. An external lightscreen used in the projection system according to claim
 3. 8. Anexternal light screen used in the projection system according to claim4.